In the recent years, we have successfully modified the Liposome/Protamine/DNA (LPD) nanoparticle formulation for targeted delivery of siRNA to the human lung cancer cells in a xenograft model. This core/shell, self-assembled nanoparticles contained a compact protamine/DNA/siRNA core which is wrapped around by two cationic lipid bilayers. When the nanoparticles were incubated with a polyethylene glycol (PEG)-phospholipid conjugate (DSPE-PEG), the outer bilayer was stripped off, but the inner bilayer survived with a high degree of PEGylation. The densely packed surface PEG formed a brush protection layer to shield the cationic charges of the nanoparticles and reduced opsonization by serum proteins. The result was a very low degree of uptake by the liver and the spleen, and a very high level of tumor uptake, up to 60-80% injected dose per g tissue. If a targeting ligand, anisamide, was tethered to the distal end of PEG, the targeted nanoparticles efficiently delivered siRNA to silence a target gene in the entire tumor. However, upon a closer look at the delivered siRNA, most of the dose was sequestered in the endosomes and was not bioavailable. This is because the formulation did not possess any endosomal lytic activity. Thus, we propose to replace the core of LPD with Ca phosphate (CaP) which dissolves at the acidic endosome pH. We expect that dissolved CaP will significantly increase the osmotic pressure of the endosome and induce swelling and rupture of the organelle, resulting in the release of the encapsulated siRNA. The proposed improvement of the non-viral vector will combine high level of tumor uptake and efficient endosome release of the siRNA cargo. The other aim of the project will deal with tumors with not-so-leaky neovasculature. We have successfully prepared small bioactive lipoplex (SBL) by mixing siRNA and a cationic lipid, DOTAP, in a fluorohydrocarbon solvent at an elevated temperature and pressure. The resulting nanoparticles were small (30-50 nm), and active in transfection. We will further modify the surface of SBL with PEGylation and ligand tethering. We expect that the new nanoparticle will deliver siRNA to tumor cells in which the neovasculature is not so leaky.